Rio de Janeiro
— When Brazil's agonizingly slow election vote count wound wearily to its conclusion this weekend, it became clear that this giant country's political map is changing more dramatically than originally thought.

Government-backed candidates won handily in 12 states, but opponents of Brazil's military-dominated government did better than pre-election polls had forecast and better than early vote counting indicated. Weekend final tallies showed that government opponents took 10 states in the Nov. 15 election - the states with the biggest share of voters.

Those voters, who had their first real taste of democracy in almost 20 years on election day, told the military leaders they should go back to the barracks.

And that is what the military appears to be doing, albeit a little more slowly than the public seems to want. The military will continue to hold the top government leadership posts at least until 1985, when a new president will be chosen.

Still, the newly elected governor of Sao Paulo State, Andre Franco Montoro, an opponent of the military-ruled government, exulted: ''This is the start of a great adventure. We are now going to implant democracy in Brazil.''

Evidently his enthusiasm was widely shared. Brazilians took to the voting process with zest - and concern.

Many Brazilians say the election results are also a tribute to Gen. Joao Baptista de Oliveira Figueiredo, the military leader of this nation of 125 million, who over the past three years has pushed the country toward democracy. The general is said to be generally pleased by the voting. He saw the elections as a vindication of that push toward democracy, which he calls abertura (opening).

''Democracy,'' he said recently, ''even one in trouble, is worth far more than any progressive dictatorship.''

But he could not have been entirely pleased with the results. Although General Figueiredo is popular with the Brazilian masses, he could not transfer his personal popularity to many of the government candidates.

Here in Rio, for example, Figueiredo campaigned hard and long for Wellington Moreira Franco, widely considered an able and attractive gubernatorial candidate of the government's Social Democratic Party (PDS). Mr. Moreira Franco did better than expected, but he fell short by 100,000 votes of populist Leonel Brizola and his newly formed Brazilian Labor Party.

The election of the left-leaning Brizola suggests to many in Brazil that there is still room here for populist leaders. Brizola himself once thought that populism was dead; that was during his 15-year exile in Uruguay, which began in 1964 when the Brazilian military deposed his brother-in-law, President Joao Goulart. Mr. Brizola was forced out of his post as governor of Rio Grande do Sul State at the same time.

But he came back, employing the sort of populist charisma that was much in evidence during the days of the late dictator Getulio Vargas from 1930 to 1945.

The Brizola populist success is a limited one, at least in the short run, for his party is only regionally based. And his triumph in Rio de Janeiro State is seen here more as a victory for Brizola the man than as a signal that populist politics will again sweep the country.

However, the major opposition party, the Brazilian Democratic Movement (PMDB) , is national in scope, and its victory in many states is viewed as trend-setting.

Opposition PMDB candidates scored some last-minute upsets as final vote counts from the frontier Acre State and the northeast Para State filtered in two weeks after the election. PMDB also appears to have won in southern Mato Grosso do Sul. It almost won in Rio Grande do Sul.

But it was the loss in both Sao Paulo and Rio de Janeiro states that is most telling. Sao Paulo is Brazil's wealthiest state, with 25 million people, one-fifth of the nation's population. Sao Paulo produces almost half of the nation's $285 billion gross national product.

Rio de Janeiro, of course, is the former capital - and remains the most favored city and the national playground, with its beaches, striking beauty, and spectacular location. Rio de Janeiro State is heavily industrialized.

General Figueiredo and his advisers are bound to find it more difficult to rule with opposition leaders in these prominent places.

But Gen. Vinicius Lemos Kruel, speaking for the armed forces, said over the weekend that the vote ''gives uncontestable proof of Brazil's political maturity.''

The official military line is one of satisfaction with the whole procedure. After all, General Figueiredo remains as president until 1985 and the federal government still holds the pursestrings of the states. The Brazilian Constitution has been remodeled to give tremendous power to the federal government, and both Mr. Montoro in Sao Paulo and Mr. Brizola in Rio de Janeiro will have to get their budgets appro ved by the federal government in Brasilia.

The military-backed PDS also controls the electoral college, which will choose the next president.

That twist on democratic practice here will surely cause some complaints in the months ahead, but it is unlikely to be altered by the military.